bug#22715: 25.0.91; Point randomly jumps during key entry

[Steve Revilak]

Thanks, Eli and Andreas, for your suggestions.

I began by collecting `view-lossage' after several instances of
point-jumping.  Here is a representative example:

     > [self-insert-command]
     SPC [self-insert-command]
     a [self-insert-command]
     l [self-insert-command]
     l [self-insert-command]
     R [self-insert-command]
     e [self-insert-command]
     q [self-insert-command]
     u [self-insert-command]
     e [self-insert-command]
     s [self-insert-command]  <<< HERE
     t [self-insert-command]
     I [self-insert-command]
     M-x [execute-extended-command]
     v [self-insert-command]
     i [self-insert-command]
     e [self-insert-command]
     w [self-insert-command]
     - [self-insert-command]
     l [self-insert-command]
     o [self-insert-command]
     <tab> [minibuffer-complete]
     <return> [minibuffer-complete-and-exit]

I'd been typing on line 494 of a .java file.  The characters "stI"
ended up on line 464.  I don't see any unexpected characters in
view-lossage; the characters that appear are those I actually typed.


Next, I tried to reproduce this behavior in a "almost emacs -Q"
configuration, which was

  emacs -q --no-site-file --no-site-lisp --no-splash

and the smallest ~/.emacs I could get away with.  I was not able to
reproduce the point-jumping behavior in this configuration.


Looking through the emacs-devel archives, I came across

  https://lists.gnu.org/archive/html/emacs-devel/2015-11/msg01687.html

which describes behavior nearly identical to what I've observed.


A later post suggests a patch

  https://lists.gnu.org/archive/html/emacs-devel/2016-01/msg00029.html

and another suggests that point-jumping might be related to timers

  https://lists.gnu.org/archive/html/emacs-devel/2015-11/msg02127.html


As an experiment, I went back to my "normal emacs", but with a few
lines in ~/.emacs commented out:

  ;(semantic-mode 1)
  ;(semantic-stickyfunc-mode t)

I also commented out `setq' definitions for
semanticdb-default-save-directory and semanticdb-project-roots.  With
those elements commented out, I couldn't reproduce the point-jumping
behavior.  (I'll continue to test in this configuration, to be more
certain that the non-reproduce isn't just a coincidence.)


If it would help, I can restore the aforementioned `semantic' pieces,
and see if I can correlate point-jumping with specific timer events.
I can also try the save-excursion patch in
https://lists.gnu.org/archive/html/emacs-devel/2016-01/msg00029.html.


Finally, 

  https://lists.gnu.org/archive/html/emacs-devel/2016-01/msg00031.html

suggests this might be related to Bug #15045.


Steve

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